1. Field of Invention
This invention relates to adaptive receiving antenna systems for simultaneously enhancing a desired signal in the output of the system while at the same time reducing interfering signals in the output of the system which have been received from directions other than that of the desired signal. This invention operates in conjunction with a method of spreading and despreading the spectrum of the desired signal wherein the bandwidth of the desired signal as received, and prior to despreading, is several times the bandwidth of the desired signal following "despreading".
2. DESCRIPTION OF PRIOR ART
In a typical adaptive antenna system, the system parameters are automatically adjusted to reduce the output from the system due to interfering signals while at the same time enhancing a desired signal. Both the prior art and this system operate to orient a portion of the antenna gain pattern with low gain, in the direction of the source of interference, and at the same time operate to orient a portion of the pattern with high gain in the direction of the source of the desired signal.
Typically, the outputs from each of the elements of an array are individually modified in magnitude and phase and then combined or summed with the outputs from the other elements to produce the output of the array. The magnitude and phase of the output from each element is adjusted automatically so as to enhance the desired signal while reducing the interfering signal in the output of the array.
The AGIPA (T. T. Noji and L. Schwartz, AIL, "Multibeam Adaptive Array for RPV Anti-Jam Communication" 1975 Convention of National Association for Remotely Piloted Vehicles, June 2-4, 1975, Dayton, Ohio) adaptive antenna exemplifies one class of prior art. As illustrated in FIG. 1, in a simplified system of this general type, the output from each antenna element is split into two parts, each of which then passes through a variable phase shifter. The output from each phase shifter then is split into two more parts, one of which is combined with the similar parts of the outputs from the phase shifters associated with the other antenna elements to produce the output of the antenna system prior to despreading. The other part of the output from each variable phase shifter passes through a 90.degree. phase shifter and is correlated with the output of the antenna system prior to despreading.
For an incremental change in the phase shift of one of the variable phase shifters, the incremental change in the output from the phase shifter occurs at right angles to the vector which represents the output, i.e., the incremental change can be represented by an incremental vector with a phase shift of 90.degree. relative to the output vector. Accordingly, the correlation of the output of the variable phase shifter, following a 90.degree. phase shift, with the output of the system prior to despreading indicates the direction in which the phase of the output of the phase shifter should be changed to reduce the output from the entire system prior to despreading.
In this type of system, the variable phase shifter is adjusted until the correlation of the output of the phase shifter, after a phase shift of 90 degrees, with the output of the system prior to despreading is reduced to zero. As a consequence of the successive adjustments of the system, the output of the system, prior to despreading, is reduced towards zero, thus removing the undesired signals from the output of the system.
If the desired signal is extracted from the undesired signals in the output of the system by spectrum despreading (as described in this application with respect to the claimed invention), a technique similar to that described above also can be used to enhance the desired signal. As illustrated further in FIG. 1, a portion of the output from each variable phase shifter, after being shifted by 90.degree., is despread and correlated with the desired signal, i.e., the despread output of the system. The phase shifter then is adjusted (in the opposite direction to that which would have been used to reduce the output of the array) to maximize the correlation of the phase shifter output, following a 90.degree. phase shift and despreading, with the output of the system following despreading. The two operations, that of decreasing the total output from the system prior to despreading and that of increasing the output of the desired signal following despreading, are combined in some manner by the decision logic indicated in FIG. 1 to achieve both goals more or less simultaneously.
A serious problem with the AGIPA type system is its inability to function in the signal maximizing mode when the undesired signals at the outputs of the phase shifters associated with each antenna element, following despreading, are significantly larger than the desired signals at the same points in the system. The invention disclosed herein utilizes measurements of the effect of a deterministic set of phase perturbations on the system to calculate phase shifter adjustments, and thereby avoids the above stated shortcoming of the prior art.